Help with drumming equipment?

Well I'll be gong to GC (Guitar Center) here in a few days and I was wondering what those little circle things are that you put around your drum heads to make them sound more deeper. I used them at guitar center once and thought they could really help with my sound. Thing is i never got the name. There kind of like a dough nut......they have a circle cut out in the middle and are usually see through and are about paper thin. Does anybody know the name?